Above answers give a precise description of what the email should look like, so just a general comment.
Profs are (usually) normal people, the fact they're higher in the academic hierarchy than you doesn't mean that they're some kind of semi-gods who smite mortals for wasting their time :). So the rules are the same as in any standard interpersonal communication (be polite and clear, use common sense, thank the person for his time etc.), the fact that somebody is a tenured professor doesn't make them a distinct kind of being that requires a special procedure. People are almost always very helpful (either because they're friendly or just flattered, it doesn't matter), the worst than can happen is not getting an answer, and even then it's usually because of lack of time. Even is prof gets annoyed by your poorly phrased email, he simply deletes it and won't take revenge ;).
The above might be obvious, but sometimes in our academic culture too much emphasis is placed on people's social/academic positions (with tenured professor elevated to some sort of semi-godlike being), which results in undergrads being scared or too shy to ask. Profs are more busy and knowledgeable, but other than that begin a prof doesn't, IMHO, entitle you to any special kind of respect (other than standard respect you should have for any person on any academic level).
Last point - it's always OK to ask, even stupid questions. Yes, you'll make a fool of yourself, but: a) the feeling only lasts for a while, b) I guess nobody will remember it (or think of you as "the one who always asks dumb questions"). It's better to be a fool for a while than to feign understanding and remain a fool.
